Multilayer thick film circuits have been used for many years to increase circuit functionality per unit of area. Moreover, recent advances in circuit technology have placed new demands on gold materials for this use. Heretofore, most of the gold materials used in multiple circuits have been conventional thick film gold compositions. These are comprised of finely divided particles of gold solids and inorganic binders dispersed in an inert organic medium. Such thick film materials are usually applied by screen printing in the pattern desired for the conductor.
Thick film materials of this type are very important and will continue to be so. However, when applying these thick film materials in patterns by screen printing, it is difficult to obtain fine line and space resolution. It is essential that all the screen printing variables such as screen quality, squeegee hardness, print speed, dispersion properties, etc., be most carefully controlled and constantly monitored to obtain good product yields.
An alternative approach is (1) to apply a layer of the gold conductive material to a substrate by means of dispersion in a photosensitive medium, (2) to expose the layer imagewise to actinic radiation, (3) to solvent develop the pattern to remove unexposed portions of the layer, and (4) to fire the remaining exposed portions of the pattern to remove all remaining organic materials and to sinter the inorganic materials.
Such an approach is found in Felton, U.S. Pat. No. 3,877,950 issued Apr. 15, 1975. This patent discloses a coating composition which comprises an admixture of:
(a) finely divided particles of gold having a size of 0.4-4 .mu.m, PA1 (b) finely divided particles of an inorganic binder dispersed in an organic medium comprising, PA1 (c) an organic polymeric binder selected from the group consisting of polymethylmethacrylates, polyethylmethacrylates and mixtures thereof, PA1 (d) a photoinitiation system, PA1 (e) photohardenable monomer, and PA1 (f) volatile nonaqueous organic solvent. PA1 A disadvantage of prior art photosensitive gold conductor compositions and particularly compositions disclosed in U.S. Pat. No. 3,877,950 is that an organic solvent is necessary to develop such material after imagewise exposure to actinic radiation, i.e., an organic solvent removes areas of the composition which have not been exposed to actinic radiation without removal of areas which have been exposed. Organic solvents often are undesirable as they may pose health and environmental hazards. There exists a need, therefore, for a photosensitive gold conductor composition which, after exposure to actinic radiation, is developable in aqueous solution. PA1 (a) finely divided particles of gold solids having a PA1 (b) finely divided particles of an inorganic binder having a glass transition temperature in the range of from 500.degree. to 825.degree. C., a surface area-to-weight ratio of no greater than 10 m.sup.2 /g and at least 90 wt. % of the particles having a size of -10 .mu.m, the weight ratio of (b) to (a) being in a range from 0.0001 to 0.25, ispersed in an organic medium comprising PA1 (c) an organic polymeric binder, PA1 (d) a photoinitiation system, PA1 (e) photohardenable monomer, and PA1 (f) an organic medium PA1 X is the residue of a chain transfer agent; PA1 Y is the residue of a di-, tri, or tetraisocyanate radical after removal of isocyanate groups; PA1 A is the residue of a basic radical which, as an entity before reaction, has a pka value of 5-14, or a salt thereof; and PA1 m and n are 1, 2 or 3, the total not exceeding 4, provided that when n is 2 or 3, only one of A need be as defined.
As set forth in this patent processing involves the coating composition being (1) screen printed onto a ceramic substrate, (2) exposed imagewise to actinic radiation to effect hardening of the exposed areas of the composition, (3) organic solvent developed to remove unexposed areas of the composition, and (4) fired in air to effect volatilizaiton of the organic medium and sintering of the inorganic binder.